How to export a .blend file to .MP4?
You don't export a blend file to a different format.
You need to render the images or frames of the animation and encode them to a video format.

In the output settings select FFmpeg video as the file format.



there will be a new tab for Encoding options.



There you can select the container and the codec for the output file.

Container is the file format for the file, (Mov, Avi, Ogg, MP4, to name a few) sometimes is referred as a "wrapper".

Codec (short for Compression-Decompression) Is how the image is compressed. The picture quality, file size and processing power needed to view the file are largely determined by the kind of codec used and the settings used for it.

For 2.8

The interface is similar but it is further divided into Encoding and Video, but the same principles apply.



If you have already rendered the images as a .png image sequence. Then all you need to do is load those images into the VSE (video sequence editor), where you can also load the sound, then set the output to FFmpeg and the video container and codec of your choosing and render.